Method and System for 4D Radiological Intervention Guidance (4D-cath)

ABSTRACT

An imaging method for radiologically guiding an instrument during medical interventions on an object comprising:
         providing a first image of said object followed by   providing updated images on-the-fly during the intervention to an operator by measuring an undersampled set of projections of said object and reconstructing said updated image based on changes between said first image or an update of said first image and said undersampled set of projections. The invention further relates to particular uses of the method and a system for radiologically guiding medical interventions on an object, comprising   means to provide a first image of the object;   an imaging apparatus measuring undersampled sets of projections;   processing means in communication with the imaging apparatus for providing updated images on-the-fly during the intervention by reconstructing said updated image based on changes between said first image or an update of said first image and said undersampled set of projections.

FIELD OF INVENTION

The present invention relates to an imaging method for radiologicallyguiding an instrument during medical interventions on an object. Theinvention further refers to a system for radiologically guiding medicalinterventions on an object.

Radiologically guided interventions are currently limited by knownimaging methods, because known imaging methods do not allow 4D imaging(3 spatial dimensions plus time), while radiological interventions are a4D process. Current means of intervention guidance are either continuousprojective imaging (X-ray fluoroscopy) or manipulate-and-shoot computedtomography (CT) imaging. Both methods leave the interventionist with ahigh degree of uncertainty regarding the position of his instruments andthe current surroundings.

Recently, there have been approaches for using magnetic resonance (MR)to guide interventions. C. O. Shirra et al. (Magnetic Resonance inMedicine 62:341-347, 2009) describe a framework for visualization ofactive catheters in 3D. In this framework compressing sensing isemployed to gain high undersampling factors. Here constraints areintroduced taking into account prior knowledge of catheter tube geometryand catheter motion over time to improve and accelerate imagereconstruction. Furthermore, given the incremental motion of thecatheter, the known position of the device of the preceding timeframe isused to penalize the reconstruction problem. In particular, the methodof Schirra et al. includes the acquisition of data with randomlyundersampled phase encodes; image reconstruction through constrainedcompressed sensing reconstruction and curve fitting the catheteroutline, which can be displayed on a high resolution roadmap.Nevertheless, MR imaging still requires special arrangements to detectand visualize the instruments used for interventions. In this respect CTis due to the nature of X-Rays easier to handle, but present CTtechniques such as CT-fluoroscopy require excessive radiation doses,preventing them from being routinely used.

For the reconstruction of X-ray CT images different reconstructionframeworks with sparsely sampled view angles have been proposed in orderto reduce the radiation dose for the patient. In G.-H. Chen et al.(Medical Physics 35 (2), February 2008) the PICCS algorithm ispresented, which reconstructs target image sequences of one measureddataset and a prior image reconstructed from the same dataset. In thereconstructed prior image the dynamic information is lost, but staticstructure in the image are well reconstructed. The prior image istherefore used to subtract the static structure from the target imagefor each time frame. A further sparsifying transform (discrete gradient)is used to further sparsify the difference. For the reconstruction ofdynamic data, the data set is further gated using the simultaneouslyrecorded electrocardiogram.

E. Y. Sidky et al. (Physics in Medicine and Biology 53, 4777, 2008) andJ. Bain et al. (Physics in Medicine and Biology 55, 6575, 2010),disclose reconstruction algorithms based on constraint total-variation(TV) minimization to reconstruct data with sparsely sampled view angles.Here, the optimization selects the image with the minimum TV amongstthose that satisfy a given constraint.

In G.-H. Chen et al. (IEEE Transaction on Medical Imaging PP, 1-1,October 2011), a data acquisition scheme and an imaging reconstructionmethod for time-resolved cardiac imaging particularly relevant to imageguidance for accurate procedural planning and cardiac functionalevaluations is described. The data acquired during a single gantryrotation of a C-arm system is reconstructed to a prior image using theclassical FDK algorithm (filtered back projection algorithm according toFeldkamp, Davis and Kress). Then the data is gated into the differentcardiac phases resulting in undersampled sets for each phase, that arereconstructed through the prior image constraint compressed sensing(PICCS) algorithm utilizing the reconstructed prior image.

H. Langet et al. (MICCIA 2011, Part I, LNCS 6891, p 97-104, 2011)discloses a method for 3D reconstruction of rotational angiography basedon an iterative filtered backprojection approach that includes asparsity constraint called soft background subtraction. This approach isparticularly useful when injecting a contrast medium particularlyleading to late vessel opacification. Z. Qi et al. (Physics in Medicine56, 6709, 2011) discloses a 4D cone beam CT method which enables thedetection of e.g. respiratory motion. The acquired data is binned intothe different respiratory phases and reconstructed using PICCS. From thereconstructed 4D cone beam computes tomography images, the motiontrajectory for an object is extracted using deformable registrationmethods.

However, there is still a need for real time CT imaging for guidinginterventions, which reduces the exposure of the patient to ionizingradiation to a minimum.

OBJECT OF THE INVENTION

The present invention aims to provide a method for true 4D imaging whichovercomes above mentioned limitations to improve radiological guidedinterventions. It is a further object of the present invention toprovide a method for fully CT guided medical intervention providingthree-dimensional information in real time during the intervention. Thepresent invention particularly aims at providing 4D CT imaging thatavoids excessive radiation dose for the patient and can be routinelyused in medical applications.

According to the invention, an imaging method for radiologically guidingan instrument during medical interventions on an object is proposed,which comprises the following steps:

providing a first image of said object followed by

providing updated images on-the-fly during the intervention to anoperator by measuring an undersampled set of projections of said objectand reconstructing said updated image based on changes between saidfirst image or an update of said first image and said undersampled setof projections.

Additionally, according to the invention, a system for radiologicallyguiding medical interventions on an object, according to the methodpreviously explained, is proposed. Such a system comprises:

means to provide a first image of the object;

an imaging apparatus measuring undersampled sets of projections;

processing means in communication with the imaging apparatus forproviding updated images on-the-fly during the intervention byreconstructing said updated image based on changes between said firstimage or an update of said first image and said undersampled set ofprojections.

The proposed imaging method and imaging system for radiologicallyguiding the instrument during medical interventions allow for dynamicreal-time imaging during the course of the intervention. Thus,volumetric data is acquired in close, timely consecution allowing theoperator to control and monitor the course of the intervention andparticularly the movement of the instrument in three spatial dimensions.The proposed method and system for medical imaging aim to provide thetemporal changes in the object to be imaged, such as instrumentmovements, by measuring a small number of projections for theundersampled sets of projections. This way, the radiation dose, thepatient is exposed to, can be reduced to a minimum, while stillproviding sufficient information on temporal changes, such as instrumentmovement.

Furthermore, the proposed imaging method and imaging system allow anoperator to monitor and hence to control the intervention. Suchinterventions can be performed on body parts, such as the cardiovascularsystem, tubular organ structures or on the brain, which exhibitcomplicated three-dimensional structures. In particular, the timecomponent, i.e. the dynamics of the intervention, play an important rolein order to ensure safe performance. This provides the interventionistwith a high degree of certainty in the way the intervention is performedand thus, reduces the risk for the patient during intervention.

Thus, the proposed method facilitates fully guided interventions basedon imaging, which is particularly relevant to interventions such ascatheter interventions, bronchoscopy interventions, implantation ofcardiac pacemakers or positioning of stents. Hence, maneuvering incomplicated three-dimensional structures becomes easier and misplacementof e.g. stents or injury due to e.g. rupture of vessels can be avoided.Overall, the imaging method according to the present invention providesfull control in three-spatial dimensions including temporal changesduring the course of the intervention.

In contrast to known systems, where the projections that are used forguidance cannot be used for tomographic reconstruction, the inventionallows to use the projection data for the reconstruction of severalpieces of information relying on the same data acquisition. Thissignificantly reduces the relative radiation costs. Additionally,through the application of tomographic imaging a lower concentration ofcontrast media is detectable.

Furthermore, the invention allows novel intervention concepts to becomefeasible. For instance, biopsies can be combined with endoscopic orintravascular accesses. The intravascular access route can be used forbiopsies of extravascular structures, e.g. pancreas biopsies through thesuperior mesenteric artery. Extrabronchial masses can be accessedthrough a bronchius, while the needle towards the mass is then imagedusing the said tomographic imaging. In particular at present, notomgraphic imaging is available, which is why the biopsie ofextraluminal lesions using endoscopes is limited to the optically,visible wall lesions.

In the sense of the present invention, imaging refers to tomographicimaging providing a volumetric image of an object as reconstructed fromone-dimensional or two-dimensional projections. A projection herebyrepresents a two-dimensional (once integrated) or a one-dimensional(twice integrated) image of a three-dimensional distribution at specificprojection solid angles of the source-detector assembly with respect tothe object to be imaged.

In particular, medical imaging is a technique and process to createimages of the human body for clinical purposes. The object to be imagedcan either comprise the full body of a patient or parts and functionsthereof. Typical imaging techniques used in this area are tomographicimaging techniques, such as magnetic resonance imaging (MRI) or computertomography (CT). In the case of MRI non-ionizing radio frequency (RF)signals are used to acquire images. CT, on the other hand, uses X-rays(a type of ionizing radiation) in order to image objects. Owing to thedose X-rays carry, only a restricted number of exposures can beperformed and the number of projections to be measured is to be kept ata minimum. However, instrument detection via X-rays is due to the natureof radiation simpler than instrument detection through MR-signals. In apreferred embodiment of the invention, said imaging method can be basedon ionizing radiation particularly X-rays, wherein the undersampled setof projections is measured with a minimized radiation dose.

The method according to the present invention provides updated images“on-the-fly”, meaning measurements, calculations and modifications takeplace without significant time delay during the intervention. Hence, themethod provides a way of real-time imaging during medical interventions.Furthermore, the real time images are provided to an operator, who canbe the interventionist or a medical robot, to further support the courseof the intervention. Lastly, an updated image represents an image thatincludes the latest temporal changes recorded during the intervention.

In the sense of the present invention, an “undersampled set ofprojections” represents a sparsely sampled set of projections, meaningthe number of measured projections violate the Shannon-Nyquist samplingtheorem. In view of the present invention, the Shannon-Nyquist theoremis in the image domain given by the highest represented frequency f. Thesampling rate must be larger than 2f to fulfil the Shannon-Nyquisttheorem. A pixel corresponds to a data point in a matrix representingthe image. In contrast to an undersampled set a fully sampled setfulfils the Shannon-Nyquist sampling theorem and includes an appropriatenumber of projections essentially uniformly distributed between 0° and(180°+ cone angle). Here essentially uniformly encompasses deviations ofup to 10%. In practical interventional radiology, the undersamplingfactor can lie in the order of 10-30 resulting in 8-35 frames perreconstruction.

In one aspect of the present invention, said first image, said update ofsaid first image and said updated images comprise volumetric images ofthe object to be imaged. Here, the first image, which includes thestatic structure of the object may be reconstructed from at least oneundersampled set of projections, at least one fully sampled set ofprojections or a combination hereof. Particularly, said first image cancomprise a high-resolution volumetric image of the object, which can beacquired prior to the intervention. Such a high resolution volumetricimage can be produced by measuring a fully sampled set of projectionsusing CT imaging or any other medical imaging modalities. Another optioncomprises to provide a high resolution volumetric image from a database,where for example previously recorded images of the object to be imagedare stored. This way a reconstruction for Prior Image Dynamic ComputedTomography (PRIDICT) can be realized allowing for real-timeinterventional guidance.

In a further aspect of the invention, a first image reconstructed froman undersampled set of projections can be improved during the course ofthe invention by incorporating any projections measured to reconstructthe updated image into the first image leading to an updated firstimage. The update of the first image thus encompasses the first imageand at least parts of one or more updated images reconstructed inprevious runs during the intervention. Previous runs in this contextcomprise any reconstructed image or equivalently any projections thathave been acquired at some time before the current updated image. Thus,the iterative method provides updated images in real-time, where areconstruction performed at a point T₂ in time may include an update ofthe first image, which again, comprises any updated images computed atearlier points in time T₁<T₂.

In another aspect of the present invention, the update of the firstimage is a sliding prior defragmenting updated images of previous runs.Thus, the update of the first image at a point T₂ can incorporateupdated images produced at any point T₁<T₂, wherein the update of thefirst image is successively updated for every reconstruction performed.In this embodiment, the projections of the update scans are preferablyacquired at projection angles that are different from earlier projectionangles such that after several rotations a new-fully sampled dataset isproduced, which can be used as update of the first image. This isparticularly advantageous when movement of the object occurs during theintervention. Such movement may e.g. include displacement of the patientrelatively to the imaging system.

In another embodiment of the present invention, not only changes, whichprogress fast in time, such as catheter movement, but also slowerchanges, such as bleedings, may be visualized by storing theundersampled sets of projections measured on-the-fly during theintervention for a delayed reconstruction of soft tissue contrast.

In another aspect of the invention, the method for reconstructing theupdated image based on changes between the first image or the update ofthe first image and the undersampled set of projections is proposed,which utilizes a compressed sensing framework. This framework isgenerally known for application in CT (see e.g. E. Y. Sidky et al.,Imaging Reconstruction in Circular Cone-Beam Computer Tomography forConstrained, Total-Variation Minimization, Phys. Med. Biol., 2008, 53,4777-4807). In general, compressed sensing allows for imagereconstruction from randomly undersampled data violating theShannon-Nyquist sampling theorem. A necessary condition for a successfulreconstruction thereby is the sparsity of the given data in anytransform domain, such as the image domain. For the highest representedfrequency f in the image domain and the sampling rate corresponding tothe number of measured projections D, image recovery is possible as longas D<2f. Assuming the projections are measured at random projectionangles, the image can be recovered from D measurements with highprobability by solving the L1 optimization problem, wherein L1 denotesthe norm defined by the sum of the absolute values in each pixel.

In one realization the reconstruction algorithm according to the presentinvention is an iterative reconstruction algorithm. In a one step ofthis algorithm the amount of changes between different images taken atdifferent points in time during the intervention can be determined byforward projecting the first image or the update of the first imageaccording to the projections comprised in the undersampled set ofprojections. Thus, a set of equivalent projections based on the firstimage or on the updated of the first image can be provided in order toperform a subtraction operation between equivalent projections of theundersampled set of projections and the projections resulting from thefirst image or its update. Here, equivalent projections refer to forwardprojected images having the same projection angles.

From the difference projections, a difference volumetric image may bereconstructed. Through the subtraction operation, static parts of theobject to be imaged, which are included in the first image or theupdated first image, are effectively subtracted from the measurementcharacterizing the updated image. Therefore, the updated image basicallyrepresents the temporal change in the object to be imaged. During, forinstance catheter interventions, this temporal change is given by themovement of the instrument.

In another implementation of the invention, the reconstruction includesan iterative minimization of a number of significant pixels, optionallyincluding further sparsifying functions, which might be applied afterreconstruction of the differences. The number of significant pixelsthereby includes all pixels signifying the temporal changes, and thus,representing the dynamic rather than the static structure of the image.Further sparsifying functions may include gradient operations, wavelettransformations, curvelet transformations, contourlet transformations ora combination thereof. Such transformations aid to reduce the number ofsignificant pixels by further sparsifying the image to be reconstructed.

In a further implementation of the reconstruction algorithm, thealgorithm itself or the scan parameters can be influenced by the amountof changes between successive measurements of projections. A combinationof different sparsifying transforms and/or one tunable sparsifyingtransforms can be used in different configurations during onereconstruction to correctly reconstruct different structures, e.g.point-like or curve-like structures. Alternatively or additionally, theweight of different sparsifying transforms can be varied e.g. accordingto the sparseness of the transformed image. In particular, the influenceby the amount of change on the algorithm can include a furthersparsification of the image, if the changes are less significant withtime.

Alternatively, the sparsification may solely be based on the differenceexcluding further sparsification, if there are significant changes intime. Significant temporal changes are, for example instrumentmovements, in contrast to less significant temporal changes, which maybe related to vessel bleedings or the like. Here, significant changescan signified by the rate of change, which may be faster than onechanging feature corresponding to one or more changing pixels in theimage per minute, preferably per 30 seconds. In contrast, a lesssignificant change may constitute less than one changing featurecorresponding to one or more changing pixels in the image per minute,preferably per 1.5, particularly preferably per 2 minutes. Furthermore,the scan parameters such as the number of projections in theundersampled set of projections and/or the dose rate per projection canbe adapted according to the amount of changes between measurementsand/or according to an input provided by the operator.

In a further implementation of the present invention, the at least onefurther sparsifying function included in the reconstruction depends onthe amount of data that has been acquired before the actual present. Inthis context, the actual present signifies a range in time, wherein thelonger ago from the present the higher the amount of low-dosetomographic data that has been acquired during the intervention. Thus ata point, where the amount of data for tomographic reconstruction onlycomprises a few projections, the amount of data is small and updatedimages can be reconstructed excluding sparsifying functions, preferablyby minimization directly in the image domain through PRIDICT (PriorImage Dynamic Computed Tomography). If the amount of data fortomographic reconstruction comprises more projections, for example datastored from previous runs, images can be reconstructed with sparsifyingfunction. In particular, updated images can be reconstructed with nofurther sparsifying function. Additionally or alternatively, thereconstruction of data stored from previous runs can include at leastone sparsifying function.

In yet a further implementation of the algorithm, the reconstruction ofthe updated image includes motion compensation. Motion compensationdescribes a method, which tracks motion in successive images andformulates the motion in terms of a transformation of one image to asuccessive image or vice versa. Such motion compensation is usuallybased on comparing corresponding features in successive images andidentifying motion vectors, which may be used to build up a motionvector field. In particular, through motion compensation periodic and/ornon-periodic motion of the object or a structure within the object to beimaged can be compensated for in the reconstruction of the updatedimages. Specifically, periodic motion compensation can be performedthrough gating measured projections into different phases of theperiodic motion and utilizing the gated projections for reconstruction.For gating the cycle of the periodic motion is typically fragmented into several intervals signifying the phases of motion and the projectionsare distributed into the corresponding intervals during measurement. Ininterventions periodic motions can result from the respiratory and/orthe cardiac motion. During the course of the intervention, respiratoryand/or cardiac motion are recorded providing a type of reference signalfor the gating. Measured projections are then distributed into thedifferent phases according to that reference signal.

In another implementation periodic motion compensation can be performedthrough a transformation mapping of images into one phase of theperiodic motion. This can be realized by applying deformations viamotion-vector fields. Examples for such algorithms are theMcKinnon-Bates algorithm or the Phase-Correlated Feldkamp algorithm. Incomparison to gating, transformation mapping enables to use all themeasured data for reconstruction, which further minimizes the radiationdose during intervention and still provides high image quality at alltime points of a motion cycle. In another implementation the motioncompensation will be performed by recording a fully sampled 4D datasetof the imaged object, e.g. the heart or the lung. From this data setmotion vectors are calculated that can then be used to support themotion compensated reconstruction.

The updated images reconstructed iteratively during the course of theintervention can be displayed in real-time on a screen allowing fordifferent representation modes which are chosen automatically or by theoperator. The image diagnostically relevant for the physician is the sumof the first image including static structures and the temporal updategiving the updated image. Therefore, the updated imaging includingstatic and dynamic structure can be provided to the operator, preferablythrough a screen. The representation of such images may for instance bepreset including either a full 3D representation of the object to beimaged and further parameters, which might be of interest duringintervention guidance. Here, the 3D representation may comprise somesolid or boundary model of the object to be imaged and in particular therelevant structures or functions therein. The representation mode can beselected from volume rendering, multiplanar reformations and all othermeans of medical image presentation. In a further version, image analyzetools are used to trace the intervention instruments in the data set andprovide the interventionist with region of interest representationsusing e.g. curved multiplanar reformation, segmentation or similartools.

Furthermore, with the updated image the operator can be provided with anindicator of the accuracy of the displayed image. This indicator ofaccuracy can depend, for instance, on the reconstruction parameters,such as matrix size or time resolution, the scan parameters, such asnumber of projections in the undersampled set, or the like. Oneindicator can be the total variation or other mean values of thedisplayed image to assess the difference between actual projections andforward projections through later reconstructed volumes.

According to the invention a computer program for performing abovedescribed method is proposed, when executing the computer program on acomputer, particularly a high performance computing device (HPC). Thecomputer program is preferably stored on a machine readable storagemedium or on a removable CD-Rom, Flash memory, DVD or USB-stick.Additionally or alternatively, the computer program is provided on aserver to be downloaded via, for example, a data network, such as theinternet or another transfer system, such as a phone line or anywireless connection.

Further according to the invention, above described method is usedduring interventions on the cardiovascular system, during catheterintervention and/or for use in the implementation of cardiac pacemakers. Further according to the invention, above described method isused during interventions on tubular organ structures, preferably lungsor kidneys, in positioning of stents in vessels or bronchi or usedduring bronchoscopy interventions. Further uses comprise interventionson the brain or angiography.

Additionally according to the invention, a system for radiologicallyguiding medical interventions on an object, according to above describedmethod, is proposed. Such a system comprises:

means for providing a first image of the object;

an imaging apparatus measuring undersampled sets of projections;

processing means in communication with the imaging apparatus forproviding updated images on-the-fly during the intervention byreconstructing an updated image based on changes between said firstimage or an update of said, first image and said undersampled set ofprojections.

Preferably, the system encompasses the necessary elements for providingthe updated image to an operator, such as screens. Preferably, thesystem comprises the components described in the context of the methodaccording to the invention in order to be capable of performing thesame. A system of this type is particularly advantageous, since itallows monitoring and controlling movements of an instrument duringmedical interventions on an object in real-time.

In a preferred embodiment of the imaging system, the imaging apparatusis a tomographic system, such as a magnetic resonance imaging (MRI)scanner or a computed tomography (CT) scanner, wherein the CT scannercomprises at least one X-ray source and at least one detector.Furthermore, the X-ray sources may differ in terms of the X-ray spectraand the detectors can provide means of energy differentiation. This waypotential material differentiation is being used to influence thereconstruction algorithm and/or the dual-energy information is beingused to extract further information on changes related to instrumentmovements.

In a further embodiment of the imaging apparatus, imaging parameters ofthe apparatus depend on the changes between said first image or anupdate of said first image and said undersampled set of projections. Theimaging parameters of the imaging apparatus can influenced automaticallyor by an operator. For the operator to monitor and control theintervention, the system can include means to provide an input and/or anoutput to an operator. Preferably, the output comprises at least oneupdated image, at least one image produced by delayed reconstruction ofsoft tissue contrast and/or an indicator for an accuracy of displayedimages.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject-matter of the invention will be explained in more detail inthe following with reference to preferred exemplary embodiments, whichare illustrated in the attached drawings. The drawings show

FIG. 1, an exemplary configuration of the 4D-CATH lab is shown (here4D-CATH stands for 4D Catheter Advancement with Tomographic Help),

FIG. 2, an illustration of a C-arm CT imaging device,

FIG. 3, an illustration of the general CT acquisition process,

FIG. 4.1, a flowchart of the general CT workflow for 4D-CATH,

FIG. 4.2, a workflow of CT image acquisition and reconstruction,

FIG. 5, an illustration of a time-dependent data acquisition,

FIG. 6, an illustration of a sliding prior,

FIG. 7, a flowchart showing the general structure of the PRIDICTreconstruction algorithm that can be used in 4D-CATH,

FIG. 8, one embodiment of the PRIDICT reconstruction algorithm as shownin FIG. 7,

FIG. 9, a further embodiment of the PRIDICT reconstruction algorithm asshown in FIG. 7,

FIG. 10.1, yet a further embodiment of the PRIDICT reconstructionalgorithm as shown in FIG. 7 including a method to influence thesparsifying function,

FIG. 10.2, a flowchart of the CT image acquisition and reconstructionincluding influencing scan and/or reconstruction parameters,

FIG. 11, yet a further embodiment of the PRIDICT reconstructionalgorithm including a further method to influence the reconstructionalgorithm,

FIG. 12.1-.3, exemplary reconstruction of the guide wire moved in apig's head,

FIG. 13, an illustration of the PRIDICT reconstruction algorithmincluding motion compensation,

FIG. 14, a flowchart for 4D intervention guidance and 3D road-mapping.

EMBODIMENTS ACCORDING TO THE INVENTION

FIG. 1 shows an exemplary configuration of the 4D-CATH lab including aCT scanner 101 in communication with a high performance computing device(HPC) 103 further in communication with one display or an array ofdisplays 102 to provide the operator 105 with imaging information forguiding the intervention. The tomography system 101 is directlyconnected to the HPC 103 like conventional clusters, GPU-systems,GPU-clusters, cloud systems or other mainframes, where the actualreconstruction of images is performed. Thus, the HCP 103 receivesprojections measured by the CT scanner 101 and sends reconstructed,updated images to at least one of the displays of the array of displays102.

In an exemplary embodiment, the CT scanner 101 comprises a continuouslyrotating, gantry-based CT scanner 101 with a flat-panel detector. Such asystem is for instance described in R. Gupta et al. (Flat-panel volumeCT: fundamental principles, technology, and applications. Radiographics.2008; 28(7):2009-2022). Other embodiments such as the CT scanner 101shown in FIG. 2 can use an alternating direction scanning C-arm or O-armscanner geometry. Compared to a gantry-based configuration such scannergeometries provide a space-saving solution, but the rotation time islimited.

During the course of the intervention the CT scanner 101 runs in acontinuously tomographic acquisition mode, while the image acquisitioncan be pulsed. Here, the first rotation can be used to run a fullysampled acquisition mode and all following rotations are performed inundersampled acquisition mode. Prior to the intervention, the priorimage can for instance be sampled using a gantry-based system with aframe-rate of 30 frames per second, a rotation time of 10 s and atube-current of 50 mA and tube voltage of 100 kV. During the course ofthe intervention, the temporal updates corresponding to undersampledsets of projections can be sampled with e.g. 18 frames per rotation, atube current of 30 mA and a tube voltage of 100 kV. However, thescanning parameters may be adjusted according to the respectiveapplication.

During intervention the patient 106 is placed within the scanner system101 and the information is provided to an operator, e.g. theinterventionist 105, through the array of displays 102. Thereby theinterventionist 105 stands next to the patient 106 and controls theintervention via the operator console 104. In other embodiments, theinterventionist 105 can also be situated in a remote location. Theoperator console 104 further allows modifying all functions of theinterventional CT system 101 and most parameters affecting the imaging,e.g. the reconstruction algorithm, are controlled by the interventionist105.

During the intervention, a CT scanner system 101 acquires images of thepatient 105. Typically, such systems comprise a source 201, 304releasing electromagnetic radiation, preferably X-rays, and a detector202, 306 detecting the released X-rays after having traversed theobjection to the image 106, 302, 310, 203. Thus, a typical result ofsuch a measurement comprises projections of a three-dimensional (3D)energy distribution. In this sense, a projection is a two-dimensional(once integrated) or 1-dimensional (twice integrated) distribution ofthe underlying 3D energy distribution at specific projections solidangles of the detector with respect to the object to be imaged.Therefore, in order to reconstruct a 3D image of the object to be imaged106, 203, 302, 310 multiple projections are measured at differentprojection angles. From the multiple of projections a 3D image of theobject to be imaged 106, 203, 310, 302 can be reconstructed.

For measuring a multiple of projections, the CT scanner 101 typicallycomprises a gantry or C-arm-based construction for rotating the source201, 304 and the detector 202, 306 around the object to be imaged 106,203, 302, 310. FIG. 2 for example shows a C-arm-based CT imaging devicewith the source 201 and the detector 202 rotating around the object tobe imaged, in this case the patient's head 203.

Similarly, FIG. 3 illustrates a top view of the C-arm scanner systemshown in FIG. 2. Both the source 201, 304 and the detector 202, 306,which is preferably closed directly opposite to the source in one linethrough the object to be imaged 106, 203, 302, 310. In order to acquireprojections at different projection angles for reconstructing 3D imagesthe source 201, 304 and the detector 202, 306 are rotated around thepatient 203, 302, 31Q. In FIGS. 2 and 3, the rotation direction isindicated by reference numerals 204, 311, 312. In fact, the general CTacquisition process includes the source 201, 304, and the detector 202,306 rotating around the patient 203, 302, 310 while measuring lineintegrals along the X-ray direction 308.

The algorithm performing the method according to the invention isimplemented on the HPC 103. During the course of the intervention, theHCP 103 calculates the updated image in real time. Here, standard CTdensity values and imaging features that are different from the standardCT values such as dual-energy index or difference images between actualprojections and forward projected data sets are used to trackinstruments and to constrain the compressed sensing criterion. Theupdated image is then provided to the interventionist by displaying theupdated image on the display array 102. For display standard graphicvolume display techniques, such as volume-rendering, surface-renderingor digitally reconstructed radiographs (DDR) are calculated from thevolumetric dataset. The DDR are for example reconstructed from variousangles which can depend on the radiologists selection or automaticallydepending on intrinsic imaging features so that the interventionguidance is optimized (e.g. perpendicular to the main movement directionof the catheter). Furthermore, angiographic features are incorporatedinto the DDR to provide a 3D road-mapping feature. All acquiredprojection images as well as all acquired temporal updates are stored inthe HPC for later use, e.g. for a later reconstruction of bleedings andother modifications in soft tissue.

Additionally, the CT-scanner 101 can employ multiple X-ray sources aswell as detectors up to arrays of X-ray sources combined with arrays ofX-ray detectors. In such an embodiment different X-ray energies can beused and the instrument can comprise material that allows detection indual-energy mode. Thus, characteristic absorption features of theinstruments with respect to multiple radiation energy can providefurther information on the instrument and its movement. This allows totrack instruments using other means than standard CT absorptionmeasurements and the detection of instruments is more robust.

FIG. 4.1 illustrates a flowchart 400 of the general CT workflow for4D-CATH as proposed according to the present invention. In a firstoptional step, a number of projections fulfilling the Shannon-Nyquistcriterion, i.e. a high number of projections is acquired at differentprojection angles. From these projections a tomographic reconstructionof a high quality prior image is performed. Herein the prior imageserves as the first image of the object to be imaged. Furthermore, theinstrument used during medical intervention on the object to be imagedis not present on the prior image. Therefore, the prior image comprisesa volumetric image of the object to be imaged with high resolution priorto the intervention. In other embodiments, such a first image may beprovided from a database, a different imaging technique or a morecoarsely sampled set of projections.

After acquisition and tomographic reconstruction of the first image, theinstrument to be guided during the medical intervention on the object tobe imaged is placed. Thus, in step 402 an instrument, such as acatheter, may be placed for further medical intervention, e.g. on theheart. After the placement of the instrument and on its way through theobject to be imaged low dose update data is continuously acquired instep 403 for guiding the instrument during the intervention.Furthermore, the low dose update data from step 403 is continuouslyreconstructed using the imaging method according to the presentinvention.

Thus after a normal dose scan, all following scans can be performed asunder sampled scans with a lower dose, which can be performedcontinuously. In practical interventional radiology, this results in anundersampling factor of the order of 10 to 30 resulting in 8 to 35frames per reconstruction. These are reconstructed 404 providing imagesthat represent temporal updates comprising changes in the examinedvolume. Update images are reconstructed using an iterative algorithm toincorporate the prior information as well as the actual temporal changesin the iterative steps. During the intervention the interventionist iscontinuously provided with updated images on-the-fly. Here the imagediagnostically relevant for the physician is the sum of the prior imageand the temporal change which is called the updated image.

FIG. 4.2 shows a flowchart 500 of the CT image acquisition andreconstruction. For the intervention in step 501 a high resolution,highly sampled CT scan is acquired as prior image. This image includesvolumetric data representing the object to be imaged. During theintervention the prior image is used as a first image incorporated inthe iterative construction algorithm during the intervention.

While performing the intervention by placing the instrument and movingit within the object to be imaged update information is continuouslyacquired in step 502. The update information comprises undersampled setsof projections, which allow for a low-dose rate. In order to reliablyreconstruct images for the physician performing the intervention, thisupdate information is incorporated with the prior image in step 503.Hence, the update information including the change of informationcorresponding to the moving instrument can be reconstructed in step 504providing updated image data. Thus, the static part of the image isprovided by the prior image, while the update information comprising aset of undersampled projections provides the temporal changes, whichcorrespond to the moving instrument. This way a Prior Image DynamicComputed Tomography (PRIDICT) may be realized allowing for real-timeinterventional guidance. Optionally, the projections measured during theintervention in step 502 may after reconstruction in step 504 be used toupdate the prior image. By the calculation of a new prior image anytemporal changes occurring during the intervention, such as movement ofpatient, can be incorporated over time into the prior image leading to ahigher image quality.

Furthermore, the update information comprising undersampled sets ofprojections may be collected in step 506 during the course of theintervention. After or during the intervention, but with a larger timedelay than for 504, the collected data sets from step 506 may bereconstructed in step 508 to visualize changes on a slower time scalethan instrument movements, such as bleedings. For this differentreconstruction algorithms can be used, including algorithms with furthersparsifying functions.

FIG. 5 shows an overview of the proposed reconstruction algorithm inrelation to the amount of data that is acquired from the actual present(514). The continuous acquisition of low-dose tomographic data duringthe intervention 510 leads to a constantly increasing amount ofprojections from a time 514 (represents present) to a time 515 in thepast. The longer ago from the present the higher the amount of low-dosetomographic data 512 that has been acquired. Thus at point 516.1, ashort time after the present, the amount of tomographic data 512 onlycomprises a few projections. Longer time ago 516 the amount of low-dosetomographic data 512 increases by increasing the number of projectionsmeasured. At a point 516.2 in time, the number of projections measuredduring the intervention, i.e. the amount of low-dose tomographic data512, is increased.

At the earlier point in time 516.1, the measured data is used 518.2 toreconstruct and display temporal changes, such as movement of guidewires or catheters 526. The reconstruction in step 521 can, e.g. beperformed by using compressed sensing, where the sparsifying is donethrough a difference with e.g. the first image.

At a later point in time 516.2, the full amount of low-dose tomographicdata 512 corresponding to the projections measured until then can beused 518.1 to visualize the anatomy, bleedings, bones, organs or otherstatic data 524. Here, the reconstruction 520 can be performed usingcompressed sensing including further sparsifying functions such asgradient functions. The sparsifying functions can thereby be selected sothat larger, more areal changes to the dataset will prevail, whileshorter, more punctual changes will not be reconstructed. Furthermore,the data reconstructed after time period 516.2 may be fed back 528 intothe reconstruction algorithm during intervention as a first image. Apartfrom the exemplary embodiments shown in FIG. 5, many more time points(such as 516.1 and 516.2) with different reconstruction algorithms,different sparsifying algorithms and different contributions to thedisplayed images exist and the person skilled in the art may adapt theexplicit execution to the specific needs of the medical intervention.

FIG. 6 illustrates how undersampled sets of projections 604 collectedduring the intervention may be defragmented for creating a fully sampledset of projections 602 which again may be used as updated first image(sliding prior). For a sliding prior, individual projections of theundersampled sets of projections 606 are culminated in accordance withtheir solid angle 608. The projections of the update scans 606 areacquired at angular positions that are different from earlier projectionpositions so that after several rotations a new-fully sampled dataset isproduced which can be used as a sliding prior. In the sliding prior, thechanges (e.g. catheters, guidewires, etc.) are removed via standardimage processing algorithms. Such algorithms can be similar to thosebeing used in metal artifact reduction, where the reconstructionincluding high contrast signals is followed by the segmentation of highcontrast signals and the elimination of these high contrast structuresin projection data via forward projection of segmented high contrastdata. The new reconstruction can then be performed without high contrastdata. Alternatively or additionally, algorithms that track instrumentscan be used. There e.g. connected pixels in the data set can beidentified, a comparison with a data base of possible instruments can beperformed or the PRIDICT (Prior Image Dynamic Computed Tomography)algorithm may even be modified so that significant pixels from theupdate will be memorized and removed. Other possibilities or additionsmay include dual-energy information e.g. through instruments, thatprovide a characteristic dual-energy signature.

FIG. 7 shows a flowchart 700 illustrating the general structure of thePRIDICT reconstruction algorithm that can be used for 4D-CATH. Theproposed algorithm PRIDICT is a reconstruction technique specialized tointerventional 3D and 4D applications. It incorporates the informationof a former scan (prior scan) to the reconstruction process and canreduce the number of relevant measurements for temporal updates farunder the Shannon-Nyquist sampling using the compressed sensingframework.

The interventional procedure starts with the acquisition of a fullysampled, normal dose scan 702 that can be used a prior image for thePRIDICT reconstruction algorithm as well as a first overview CT scan forthe physician. This fully sampled scan can be reconstructed in step 704through a standard CT reconstruction algorithm such as the FDK (FeldmannDavis Kress, as for example explained in Feldkamp L A, Davis L C, KressJ W. Practical cone-beam algorithm. J. Opt. Soc. Am. 1984; 1(6):612-619)to provide a prior image 706. The prior image 706 forms the basis image708 for the iterative PRIDICT reconstruction algorithm.

During the intervention undersampled sets of projections 712 aremeasured. These provide the update information including static as wellas dynamic components of the object to be imaged. In a first step of thealgorithm, the image 708, which in the first iteration is equal to theprior image and includes volumetric data, is projected in accordancewith the projection angles measured in the undersampled set ofprojections 712. The projected data 710 from the image 708 is thensubtracted individually from the update projections of the undersampledset of projections 712 in operation 714.

The subtraction in operation 714 leads to difference images 716, whichrepresent the difference between the undersampled set of projections andthe projected prior image. These difference images 716 are reconstructedthrough a standard reconstruction routine in CT such as FDK to provide areconstructed and fully volumetric difference image 718. In operation720, the image 708, which in the first run corresponds to the priorimage 706, is added to the volumetric difference image 718. In operation722, image 706, which is the prior image and stays the prior image forevery iteration, is subtracted. In the following steps 724, variousimage processing and mathematical operations, such as optimizationroutines, may be used to modify the image. This image is fed back intothe iterative loop and serves as the base image 708 for the nextiteration of the reconstruction algorithm.

In FIG. 8, the PRIDICT algorithm 800 is illustrated including aminimization loop in place of the mathematical operations performed instep 724 of FIG. 7. Equivalently to FIG. 7 a fully sampled set ofprojections 802 is measured, reconstructed via the FDK algorithm 804 toa volumetric data set 806, which is fed into the image 808 forming thebasis of the PRIDICT algorithm. Then difference images 818 arereconstructed in 816 from the subtraction of the projections of theundersampled set of projections 812 and the projections projected fromthe image 808 in step 814. Then image 808 is added in step 820 followinga subtraction of prior image 806 in step 822. Thus, the resulting image824 excludes any static components and highlights the change happened inthe intervention during the measurement of the undersampled set ofprojections 812.

In place of step 724 in FIG. 7, the embodiment of FIG. 8 basicallyincludes a minimization routing minimizing the number of significantpixels. The temporal updates are calculated as the FDK(Feldkamp-Davis-Kress) reconstruction of the difference of the actualmeasured projections and the calculated forward projections of the priorimage. These FDK reconstructions contain only information of the currentchanges in the image but include a large number of streaking artifacts.To reduce these streaking artifacts, the total number of significantpixels (represented by the L0 norm) has to be minimized. Mathematically,the minimization of the L0 norm is difficult, so the L1 norm can beminimized alternatively, e.g. by using the method of the steepestgradient, other convex optimization techniques can also be used. Withoutconstraints, the global minimum would be a zero matrix; however inpractice this would imply no changes in the volume so that the priorimage and the current image are identically. To eliminate the streakingartifacts without clearing the whole update, the minimization step hasto be adjusted to the FDK reconstruction step, so that raw datacongruence is aimed. This is presented in the next paragraph.

Without constraints, the global minimum of the L1 norm would be a zeromatrix, but in fact this would eliminate any information in the updateimage. The link between the minimum number of independent probes and thenumber of significant pixels in the image: m≈S ln(N) where N×N is thesize of the reconstruction matrix, S is the number of significant pixelsand m is the number of independent probes. Using this context, themaximum number of significant pixels in an image can be calculated forevery given acquisition scenario. We call this context the CSC(compressed sensing criterion). The minimization process is continued aslong as the CSC is not fulfilled. As soon as the L0 norm is smaller thanthe calculated maximum number of significant pixels, the CSC is reached,the minimization stops and the next iteration is performed. The L1 normhas not to be minimized directly, even optimizations minimizing L1casually might be useful.

As described, L1 is minimized because of the mathematically difficultiesminimizing L0. The actual aim is to minimize L0, so in an embodiment ofthe algorithm, other optimizations minimizing L0 casually or directlymay be used, even if they do not minimize L1.

Within the minimization loop after operation 822, the signum of theimage 824 including only the temporal changes is calculated in 826 andsubtracted from the image 824 in operation 828. From there it can bechecked whether the compressed sensing criterion 830 is fulfilled. If itis fulfilled, the image is fed back into 808 replacing this image andimage 808 can be displayed to the operator. If the compressed sensingcriterion in 830 is not fulfilled, the image will be fed back into thecompressed sensing minimization loop to 824.

In contrast to the reconstruction algorithm shown in FIG. 8, FIG. 9includes a further sparsifying transform 936 performed before theminimization of the number of significant pixels. As sparsifyingtransform 936 gradient operations, wavelet transformations, curvelettransformations, contourlet transformations or a combination thereof canbe used. This transform is then applied after reconstructing thedifference 918 and before the minimization loop is entered in 924.

In step 936 it is also possible to use a combination of differenttransforms and/or tunable transformations, which can be used indifferent configurations during one reconstruction, to correctlyreconstruct different structures point-like or curve-like structures.Furthermore, the weight of different sparsifying transforms can bevaried e.g. according to the sparseness of the transformed image.

The realization of such a tunable PRIDICT reconstruction algorithm 1000is shown in FIG. 10.1. The algorithm shown in FIG. 10.1 corresponds tothe algorithm shown in FIGS. 7 to 9. The embodiment of FIG. 10, however,incorporates the image processing and mathematical operations 1024including the minimization loop 1028 for finding an optimal differenceimage and a pre-applied sparsifying function 1026. Here, the sparsifyingfunction as well as the minimization loop 1028 may be influenced by theactual difference 1030 given in the static prior set of projection 1002and the undersampled set of projections 1012 including the temporalchanges. So certain parameters of the mathematical functions, i.e. thesparsifying function and the minimization, may be modified in accordancewith a comparison 1030 done between the prior set of projections 1002and the undersampled set of projections 1012. Thus, the reconstructionalgorithm PRIDICT 1000 can be tuned and adapt to different situationsdepending on the temporal changes. This is particularly advantageouswhen reconstructing temporal changes from guide wires, catheters, tubesand the like or temporal changes which move much slower such asbleedings.

FIG. 10.2 shows further possibilities for influencing the reconstructionof updated images. The flowchart 1040 comprises in step 1042 theacquisition of undersampled imaging data during the intervention. Fromthis data and the previously acquired prior image or an updated priorimage the amount of difference and/or movement in the examined volume iscalculated in step 1044. Then the calculated amount of difference and/ormovement can be used to influence scan parameters and/or reconstructionparameters, such as the number of projections included into theundersampled set of projections, in step 1046. Optionally, suchinfluences can also be triggered from the outside such as an operator1052. Furthermore, these influences are used to vary the reconstructionalgorithm 1048, such as reconstruction matrix size, time resolution,sparsifying function and so on. Lastly, a marker for the degree ofcompleteness of the imaging data may be provided in operation 1050.

In order to provide a parameter of completeness, FIG. 11 shows anotherrealization of the PRIDICT reconstruction algorithm 1100, which broadlycorresponds to the reconstruction algorithm PRIDICT shown in FIGS. 7 to9. Here the subtraction 1114 is not only used to calculate thedifference image 1116 but also to calculate the total modification 1120in the image. From there, the matrix size can be set 1122 which can befed back to the difference reconstruction image 1118. This results inoptimized reconstruction minimization of the significant number ofpixels. Thus, if there is a large change the modifications are large andthe matrix size may be smaller. On the other hand, if the changesbetween the prior image data and the undersampled set of projections aresmall, the modifications are small and the matrix size may be set to alarger value in order to reconstruct the difference properly.

FIG. 12 illustrates the effect of the proposed PRIDICT reconstructionalgorithm on a head. A pig head was scanned prior to the intervention,the result of which is shown in FIG. 12.1. After the insertion of aguide wire into an arteria a temporal update was provided. Here, FIG.12.2 shows the reconstruction of the undersampled set of projectionsusing a standard FDK algorithm. In comparison to FIG. 12.1 showing theresult reconstructed by a standard FDK algorithm of the fully sampledprior image, FIG. 12.2 includes streaks and artifacts. In FIG. 12.3, theresult including the guide wire is shown reconstructed through thePRIDICT reconstruction that uses a fully sampled prior image and anundersampled set of projections. As can be seen, the result of FIG. 12.3is artifact-free and the magnified inset shows the wire labeled throughthe smaller arrow and the vessel through the larger arrow.

FIG. 13 illustrates different embodiments 1302, 1306, 1304 for motioncompensated reconstruction in connection with the proposed PRIDICTreconstruction algorithm. In the first column 1300 of FIG. 13, a priorimage 1308 reconstructed from a fully sampled set of projections 1312includes different phases 1310 of the cardiac motion. As a consequenceof the cardiac motion, its appearance changes from projection toprojection. Therefore, the prior image 1308 results in a smeared imageof the heart 1322, which is smeared by the heart motion during themeasurement. The bottom part of column 1300 illustrates thecorresponding undersampled set of projections where differentprojections represent different cardiac phases 1318 as well. Here againthe heart 1318 periodically changes appearance for the different cardiacphases 1318 from projection to projection 1316.

Column 1302 illustrates one implementation for motion-gatedreconstruction. Under the assumption that the heart appearance is thesame for a single cardiac phase over the entire scan time, a gating isperformed to assort the acquisition into the heart cycle which itself isdivided into phase bins with sufficiently small widths. Thus, thedisplacement of the heart is taken into account for e.g. each projectionmeasured in the undersampled set of projections. In the exampleillustrated in column 1302, bottom part, the undersampled set ofprojections 1316 is binned into three different phases of the heart beatcycle 1324, 1326, 1327. These phases may for instance be monitoredthroughout the intervention, e.g. through electrocardiography, and thebinning is carried out in accordance with the monitored referencesignal. Incorporating the prior image 1322 using the PRIDICT algorithmto reconstruct the heart in each individual phase 1324, 1326, 1327results in update images which show the instrument 1313 in theequivalent place for each cardiac phase 1324, 1326, 1327. This way, thesmearing due to the heart motion can be compensated for and the temporalchange due to the instrument 1313 can be reconstructed according to thecardiac phases 1324, 1326, 1327. Thus, the interventionist can at eachpoint during the intervention assess where within the heart theinstrument is situated. Owing to the reduced smearing resulting from thecardiac motion a more accurate position of the instrument 1313 withinthe heart can be displayed to the operator. The advantage of such analgorithm is that there is no gating signal necessary for the priorimage.

In column 1306 another implementation of motion-compensatedreconstruction via PRIDICT is illustrated. In this case the prior image1308 is binned into the cardiac phases 1334, 1336, 1338 rather than thereconstructed update image 1340. Here no gating signal is necessary forthe reconstruction of the heart phases and thus, slower scanner systemsmight be utilized.

Column 1304 shows another implementation of motion-gated reconstructionthrough PRIDICT. Here, rather than reconstructing the time frames withrespect to cardiac and/or respiratory phases or the prior as shown incolumns 1302, 1306, the prior as 1308 well as the time frames 1314 maybe reconstructed with respect to the cardiac and/or respiratory phaseusing compressed sensing reconstruction. In this embodiment, gating isnecessary for the prior as well as the update image and images can bereconstructed with less motion-related smearing.

Furthermore, the reconstruction scan can be incorporated with low doseupdate scans using motion-compensated reconstruction combined withcompressed sensing and taking a 4D representation of the moving heartinto consideration. The idea of motion-compensated 4D reconstruction mayalso be deeply integrated into the reconstruction algorithm. In order todo so, the cardiac and/or respiratory phases are registered or thetransformation may be done through morphing or movement field. By usinga transformation the image may be projected into either the moving spaceor a static space. In a static space the object to be imaged may bedisplayed in one phase only, which is particularly useful for theguidance of catheters. Furthermore, the requirements to the scanningspeed are relaxed. With appropriate motion-compensating reconstructionalgorithms (including movement vector fields) the data that is acquiredat a certain heart phase can be used to reconstruct images at adifferent heart phase.

FIG. 14 shows an overview illustrating the capability of the PRIDICTreconstruction algorithm. Starting from a full dose prior image 1410 ofthe object to be imaged during intervention with a moving instrument lowdose updates 1412 are acquired. From these low dose updates a 4D dataset of the moving instrument 1414 may be reconstructed. Furthermore, thelow dose updates 1418 may be used to reconstruct the 3D data set of abody part with intervascular contrast media 1420. Lastly, the 4D dataset of the moving instrument 1414 and the 3D data set of the vascularstructure 1420 can be merged to a 4D intervention guidance in 3Droad-mapping 1416. Thus, the movement of the instrument during theintervention may be visualized after the intervention in the form of aroadmap illustrating the full course of the intervention.

The present invention relates to an imaging message for radiologicallyguiding an instrument during medical interventions on an object usingX-rays. The invention further refers to a system for radiologicallyguiding medical interventions on an object according to the proposedmethod.

Without limiting the foregoing, further embodiments are briefly linedout in the following:

In one embodiment, the method for performing imaging during radiologicalinterventions, comprising the steps

Measurement of more than one set of projections of an object to beimaged at different points in time, wherein the measurement is performedby an imaging apparatus,

Dynamic reconstruction of volumetric datasets from the more than one setof projections, wherein the reconstruction is performed by processingmeans.

The method according to the present invention allows for dynamicimaging. In dynamic imaging, volumes are acquired in close, timelyconsecution. Here the term timely is used in the sense temporal orequivalently in the sense of different points in time. Volumetricdescribes that image data sets are reconstructed in 3 spatialdimensions. In combination dynamic volumetric imaging describes 3D+timeimaging or 4D imaging respectively. Dynamic imaging thus allows forcontrolling and monitoring interventions in three spatial dimensions.Such interventions are preferably performed on body parts, such as thecardiovascular system, tubular organ structures or on the brain, whichexhibit complicated three dimensional structures. Furthermore, the timecomponent, i.e. the dynamics, of interventions, such as in catheterinterventions, bronchoscopy interventions, implantation of cardiacpacemakers or positioning of stents, plays an important role in orderassure save performance interventions. With the proposed method fordynamic imaging temporal changes in the object to be imaged, such asinstrument movements, can be controlled and monitored during theintervention. This provides the interventionalist with a high degree ofcertainty in the way the intervention is performed and thus reduces therisk for the patient during intervention. In particular maneuvering incomplicated three dimensional structures becomes easier and misplacementof e.g. stents or injuries due to e.g. rupture of vessels can beavoided. Overall the imaging method according to the present inventionprovides full control in three spatial dimensions including temporalchanges during interventions.

In contrast to prior known intervention guidance techniques, which arenot 4D and leave the interventionalist with certain degree ofuncertainty regarding the position of the instruments and the patientcondition, the systems and methods of the present application embody amethod of 4D intervention guidance (named 4D-CATH which stands for 4DCatheter Advancement with Tomographic Help).

In one embodiment of the invention, the imaging is a tomographic system,such as a magnetic resonance imaging (MRI) scanner or a computedtomography (CT) scanner. In the case of MRI non-ionizing radio frequency(RF) signals are used to acquire images. CT, on the other hand, usesX-rays (a type of ionizing radiation) in order to image objects. Owingto the dose X-rays carry, only a restricted number of exposures can beperformed and the number of projections to be measured is to be kept ata minimum. Preferably, the imaging system utilized in connection withthe present invention is a flat-panel cone-beam CT system and a CTreconstruction method to calculate 3D data of the examination volume.

The examination volume is the volume that is being imaged or any part ofit. The maximum size of the examination volume is limited due to thefield of measurement of the tomographic system. Examination volume caninclude the patient and instruments in it. The examination volume isdefined by physical characteristics of the scanner system.

In one embodiment the CT data (CT fluoroscopy) are continuously acquiredto provide 4D information. The systems and methods of the presentapplication as applied to intervention guidance provide 4D imagingduring interventions. Some embodiments reduce the radiation dose duringCT fluoroscopy, reduce imaging artifacts and provide the radiologistwith 4D imaging data.

Exemplary embodiments of the present application can comprise a CTreconstruction algorithm based on compressed sensing. Or thereconstruction can be based on McKinnon-Bates, PICCS or TRI-PICCS.Furthermore, in one embodiment the underlying CT reconstructionalgorithm is based on prior knowledge of the scanned volume.

Certain embodiments employ the prior knowledge of a prior data set thatis updated with few CT projections during intervention. The update ispreferably derived with a few projections or very low-dose projections.The reconstruction to combine a prior scan and the update informationcan preferably be an iterative CT reconstruction method, which can bebased on the theory of compressed sensing.

The prior data and update information can be acquired over a limitedangle orbit around the patient. The reconstruction algorithm can useinformation that it derives by comparing the update scan with the priorscan to correct for limited angle CT reconstruction artifacts ordistortions. This can be done by calculating a local distortionparameter by comparing the sparsely sampled limited angle updates withthe well sampled prior scan.

In one embodiment of the invention the continuously acquired data setsare collected and reconstructed from data collected throughout variousamount of rotations, sampling rates and hence with a different timeresolution and image qualities (SNR, soft-tissue contrast, etc.). Theposition of the projection positions might vary between rotations toacquire highly sampled data sets from several rotations.

In one embodiment the system provides the radiologist with informationabout the degree of completeness of the data. In one embodiment thismight be realized through variations in the reconstruction matrix size.

One embodiment includes a system to track the interventionalists handsand avoid the radiation of the hands when they cross the direct X-raybeam area.

One embodiment employs a registration algorithm that compensates formovements of the patient or system between acquisition of the prior scanand the update scans. In one embodiment a new prior image is constantlyupdated from the projection data.

One embodiment includes iterative reconstruction algorithms, where theresult of a former reconstruction (f−1) is used as the initial input ofthe actual reconstruction (f). Alternatively, an assumed threedimensional structure can be used as a first guess.

In one embodiment the changes within the examined volumes are measuredduring intervention and the amount of changes influence the scan andimaging parameters (e.g. the number of projections used for the temporalupdate, the temporal resolution, the number of reconstructions shown tothe interventionalist, the used sparsifying function, the X-ray tubecurrent, the voltage, etc.). The amount of changes is a parameterproportional to the significant pixels; this can be the number of pixelsthat are different in the prior image and the temporal update. However,the amount of changes can also be proportional to the number ofsignificant pixel in any domain that was used by the sparsifyingfunction. In one version the amount of changes are measured by comparingprojections of the PRIOR with projections of the update scan; in oneversion of the embodiment, the amount of changes between consecutivescans are measured by calculating the difference images, in one versionmeasured projections are compared with forward-projected projections. Inone embodiment the amount of dose that is applied will be influenced bythe amount of difference in the timely updates. In one embodiment theradiologist will get information about the degree of completeness of thereconstructed scan data. The uncertainty in the information will beexpressed through a display or uncertainty in the reconstructiondataset.

In one embodiment the algorithm incorporates information about theposition of vessels from angiography in the reconstruction process. Inone embodiment this is used to calculate a likelihood of vesselperforation, to even further reduce the amount of necessary projectionsfor guide-wire guidance by assuming the most likely position ofinstruments will be within vessels.

In one embodiment multiple X-ray sources work at different energies.Potential material differentiation is being used to influence thereconstruction algorithm. In one embodiment the dual-energy informationis being used to which information changes are related to instrumentmovements.

In one embodiment 4D digital subtraction angiographies are reconstructedto provide a 3D road map for intervention guidance. In one embodimentthe 4D angiography is reconstructed by combining a highly-sampled priorscan without contrast media with sparsely-sampled updates that areacquired during the contrast media injection.

Yet another embodiment relates to a computer readable medium havingrecorded instructions which, if executed by a computer, would cause amethod to be performed. Such a method comprises (a) capturing aplurality of images; (b) optionally acquiring a prior imaging and timelyupdates during radiological interventions; and (c) combing the timelyupdates with the PRIOR, potentially using iterative CT reconstructionmethods potentially based on compressed sensing.

Both the foregoing summary and the following brief description of thedrawing and the detailed description are exemplary and explanatory andare intended to provide further details of the compositions and methodsas claimed. Other objects, advantages, and novel features will bereadily apparent to those skilled in the art from the following detaileddescription.

FIG. 1 shows an exemplarily configuration of the 4D-Cath-Lab with aclosed gantry for continuous rotations (101), an array of displays(102), both connected to a high performance computing setup (103). Allfunctions of the interventional CT systems and most parameters affectingthe reconstruction can be controlled using the operator console (104).The position where the interventionist stands is marked as 105.

FIG. 2 is an illustration of the image acquisition phase of a dynamicinterventional C-arm CT imaging device with source (201) and detector(202) are rotating around the patient (203).

FIG. 3 is an illustration of the general CT acquisition process with acircular trajectory (302), the source (304) and detector (306) rotatingaround the patient (310) while measuring line integrals along the X-raydirection (308).

FIG. 4.1 is the flowchart of the general CT workflow for 4D-CATH.

FIG. 4.2 is a flowchart of CT image acquisition and reconstruction.

FIG. 7 is a flowchart showing the general structure of the PRIDICTreconstruction algorithm that can be used in 4D-CATH.

FIG. 8-11 are potential variations of the PRIDICT algorithm.

FIG. 10.1 is an example of a method to influence the sparsifyingfunction and the update reduction function depending on the amount ofdifferences in the update and prior projections.

FIG. 5 illustrates one of the possible modifications to the proposedalgorithm collecting projection and reconstruction datasets for adelayed reconstruction of soft tissue contrast.

FIG. 10.2 is a flowchart showing a modification of the algorithm inwhich the continuous data acquisition process and image reconstructionalgorithm is varied with respect to the actual differences/movements inthe acquired volume.

FIG. 10.2 Workflow to integrate the radiologists' input to influence thedegree of changes to the image acquisition and reconstruction algorithmaccording to the amount of changes in the examined volume.

FIG. 12 shows exemplarily reconstructions of a guide wire moved in apig's head. Fully sampled standard reconstruction is compared to theundersampled standard reconstruction as well as to the PRIDICTreconstruction using fully sampled prior and undersampled temporalupdate. The prior is a fully sampled dataset reconstructed FDK(Feldkamp-Davis-Kress) algorithm (902). The update is a factor 20under-sampled CT dataset of the same volume, but with the guide wireinserted, standard FDK reconstruction leads to 904, reconstruction ofprior together with update using PRIDICT results in 906.

One of the preferred embodiments of the algorithm is a gantry-based CTscanner system with a flat detector-based imaging chain (FIG. 1).Another embodiment consists of a C-arm or O-arm scanner geometry (FIG.2).

The algorithm can in general be used with any kind of imaging modalitywith a continuous tomographic acquisition.

A continuously rotating CT gantry is the preferred implementation bute.g. alternating direction scanning C-arm systems can be used as well.Furthermore, various scanning trajectories can be incorporated.Additionally, multiple X-ray sources as well as detectors up to arrayX-ray sources combined with array X-ray detectors could be employed.

The system can be run in a continuously tomographic acquisition mode andthe image acquisition can be pulsed. In a potential implementation usinga continuously rotating CT gantry, the first rotation would be run infull sampled acquisition; all following rotations are performed with afraction of projections acquired.

The tomography system can be directly connected to a high performancecomputing setup (HPC) like conventional clusters, GPU-systems,GPU-clusters, cloud systems or other mainframes.

The algorithm can be implemented into the HPC system; the systemcalculates the current image to be shown to the interventionist anddisplays it in various ways on the display array. This can happen inreal-time. Standard graphic volume display techniques can be employed,such as volume-rendering, surface-rendering, etc. In one embodiment DDR(digitally reconstructed radiographs) are calculated from the volumedataset. In one embodiment the DDR are reconstructed from various angleswhich can depend on the radiologists selection or automaticallydepending on intrinsic imaging features so that the interventionguidance is optimized (e.g. perpendicular to the main movement directionof the catheter). In one embodiment angiographic features areincorporated into the DDR to provide a 3D road-mapping feature.

All acquired projection images as well as all acquired temporal updatesmay be stored in the HPC for later use, e.g. for a later reconstructionof bleedings and other modifications in soft tissue.

The proposed Algorithm PRIDICT (Prior Image Dynamic Computed Tomography)(FIG. 7 or 8) is a reconstruction technique specialized tointerventional 3D and 4D applications. It incorporates the informationof a former scan (prior image) to the reconstruction process and canreduce the number of relevant measurements for temporal updates farunder the Shannon-Nyquist sampling using the compressed sensingframework.

The prior scan may be fully sampled, meaning the number of measuredprojections comply with the Shannon-Nyquist theorem, while the temporalupdates are highly undersampled and do not satisfy the Shannon-Nyquisttheorem. In the following, the algorithm is described in three steps:

Image acquisition and processing including the reconstruction are shownin FIG. 4.1, 4.2, 5. The interventional procedure starts with theacquisition of a fully sampled, normal dose scan that can be used asprior image for the PRIDICT reconstruction algorithm as well as firstoverview CT scan for the physician (501). After that normal dose scan,all following scans can be performed as undersampled scans with a lowerdose (403), which can follow different trajectories (examples in FIG.12). In practical interventional radiology, this could be anundersampling factor in the order of 10-30 resulting in 8-35 frames perreconstruction. These reconstructions are called temporal updates (403),because they represent only the changes in the examined volume. Theseupdate images are reconstructed using an iterative algorithm toincorporate the prior information as well as the actual temporalinformation in every iterative step (503). The image diagnosticallyrelevant for the physician is the sum of the prior image and thetemporal update which is called current image.

The raw data-based image reconstruction can be divided in two separatecategories, the reconstruction of the prior image and the reconstructionof the temporal updates.

The reconstruction method of the prior image in generally is independentof the rest of the algorithm, every algorithm can be used assuming thatthe reconstructed prior image is free of artifacts and not dominated bynoise.

The temporal updates are calculated as the FDK (Feldkamp-Davis-Kress)reconstruction of the difference of the actual measured projections andthe calculated forward projections of the prior image. These FDKreconstructions contain only information of the current changes in theimage but include a large number of streaking artifacts. To reduce thesestreaking artifacts, the total number of significant pixels (representedby the L0 norm) has to be minimized. Mathematically, the minimization ofthe L0 norm is difficult, so the L1 norm can be minimized alternatively,e.g. by using the method of the steepest gradient, other convexoptimization techniques can also be used. Without constraints, theglobal minimum would be a zero matrix; however in practice this wouldimply no changes in the volume so that the prior image and the currentimage are identically. To eliminate the streaking artifacts withoutclearing the whole update, the minimization step has to be adjusted tothe FDK reconstruction step, so that raw data congruence is aimed. Thisis presented in the next paragraph.

Regularization of the minimization process. Without constraints, theglobal minimum of the L1 norm would be a zero matrix, but in fact thiswould eliminate any information in the update image. The link betweenthe minimum number of independent probes and the number of significantpixels in the image: m≈S ln(N) where N×N is the size of thereconstruction matrix, S is the number of significant pixels and m isthe number of independent probes. Using this context, the maximum numberof significant pixels in an image can be calculated for every givenacquisition scenario. We call this context the CSC (compressed sensingcriterion). The minimization process is continued as long as the CSC isnot fulfilled. As soon as the L0 norm is smaller than the calculatedmaximum number of significant pixels, the CSC is reached, theminimization stops and the next iteration is performed.

The L1 norm may not be minimized directly, even optimizations minimizingL1 casually might be useful.

As described, L1 is minimized because of the mathematically difficultiesminimizing L0. The actual aim is to minimize L0, so in an embodiment ofthe algorithm, other optimizations minimizing L0 casually or directlymay be used, even if they do not minimize L1.

In one embodiment of the algorithm, wavelets, curvelets or contourletscan be used for the sparsifying transform. In embodiment the sparsifyingfunction might be applied after reconstruct the differences (FIG. 9).

In another embodiment of the algorithm, a combination of differenttransforms may be used and/or one tunable transformation can be used indifferent configurations during one reconstruction to correctlyreconstruct different structures as point-like or curve-like structures.

The weight of different sparsifying transforms can be varied e.g.according to the sparseness of the transformed image.

Accordingly, one method of the present application entails guidance ofcatheters, tubes, syringes, needles, guide-wires, coils, stents, vesselprosthesis and similar devices. Similarly intervention devices such asbiopsy needles, drainage needles, catheters, thermo ablation andchemoembolization devices are imaged. Potentially visible embolizationagents are visualized as well as contrast media of different kind.

A combination of all acquired projections or a part of them can be usedfor a reconstruction of soft tissue images and/or changes, intracranialhemorrhage etc. This feature can be used for an intra-operativedetection of possibly vessel damages. Therefore, the single temporalupdates can be reprojected. Out of measured and calculated projections,a stationary projection without any moving structures can be calculated(e.g. by subtracting moved parts from the measured projections).

A function to adopt the spatial resolution in the image (the imagematrix) to the number of samples and the changes in the examined volumecan be included. A method to calculate the changes and the sparsity inthe used image domain is required, one possibility for that can be sumof squared differences of measured update projections and calculatedforward projections (FIG. 11).

Preferred workflow looks as follows: The patient is placed in acontinuously rotating CT gantry and scanned once with high dose and fullsampling when the surgery starts. While the physician moves the guidewires and catheters, the CT system acquires undersampled CT datasets.The result of the reconstruction using a PRIDICT-based algorithm isdisplayed directly on the displays in different visualizations (FIG.4.2).

FIG. 3 schematically depicts an exemplary imaging apparatus 300. Imagingapparatus 300 has a gantry 302 to carry source and detector devices 304and 306, with a hollow center to allow the insertion of a test subject310. Rotatable around the gantry 302 in the present example is a source304, which can be, for example, an X-ray source for X-ray CT imaging.Source 304 produces radiation which travels in exemplary fashion alongpath 308, to reach detector 306. The radiation passes through subject310. Thus, detector 306 records the intensity of the transmittedradiation.

Both source 304 and detector 306 may be rotated about gantry 302 tocapture a plurality of images at varying angles θ, which may be measuredfrom any cross-sectional line through the hollow 302. To be rotatedabout means to be positioned at more than one angle θ along, allowingthe acquisition of two or more images at different angles.

Imaging apparatus 300 may preferably be a CT scanner (e.g. VCT, SiemensMedical Solutions, Forchheim, Germany), which includes a flat detectorand a modified X-ray tube both mounted on a multi-slice CT gantry. Theflat detector (PaxScan 4030CB, Varian Medical Systems, Palo Alto,Calif.) employed by this scanner consists of 2048×1536 detector pixelson an active area of 40×30 cm² resulting in a pixel size of 1942 μm².X-ray photons are converted to light photons in a scintillation layer ofthallium-doped caesium-iodine crystals which are subsequently detectedby the photodiodes of the amorphous silicon wafer.

The X-ray tube may be modified to increase the anode angle to increasethe cone angle of the generated X-ray beam. The focal spot size can bereduced to 0.5 mm to decrease the effect of penumbral blurring. An evensmaller focal spot is technically feasible. This may result in a reducedphoton flux and could make advisable longer exposure times.

Taking the geometry of the present example of the scanner intoconsideration, the scanner's total scan field-of-view is 25×25×18 cm³.The active detector area may be reduced to 192 lines in z-direction and1024 rows in x-y-direction to increase frame rate. The detector can beread out in a 2×2 binning mode, meaning that four neighbouring pixelsare averaged. This results in a decreased scan field-of-view of25×25×4.5 cm³, which is still large enough to cover the entire thoraxand diaphragm of a rat. The resulting frame rate is 100 frames persecond (fps), which translates into an exposure time of 10 ms perprojection.

The spatial resolution of the preferred scanner mentioned above, ascomputed by scanning a tungsten wire phantom, is 24 lp/cm at 10%modulation transfer function. This isotropic spatial resolutiontranslates into a minimal detectable feature size of 200 μm. Eachprojection image acquired during gantry rotation is time stamped and islabeled with the angle of acquisition. Scanner rotation times can bevaried from 2 s to 19 s in steps of 1 s, the maximum total scan timebeing 80 s. The tomographic image reconstruction is based on a modifiedFeldkamp algorithm.

A tube voltage of 80 kV and a tube current of 50 mA with continuousradiation may preferably be selected.

Various aspects of the present application will be illustrated throughexamples depicted in FIGS. 3-8. In FIG. 4, an overall method forgenerating intervention guidance image data according to the presentapplication is depicted.

In a preferred embodiment, continuous rotations are carried out, withone rotation being performed in 5 seconds. One hundred projections aregenerated per second, and each rotation is a full 360 degrees. Thus,there can be 500 projections per rotation, and 8000 per total imagingtime. The amount of projections acquired can be substantially reduced.During intervention guidance, the prior is acquired with 500 projectionsand for 4D intervention guidance a 2 second rotation time is used,together with 20 frames per second. Using PRIDICT or a variation of it,the continuous updates will be reconstructed.

The effect of the PRIDICT reconstruction algorithm is visualized in FIG.9. A pig corps' head was scanned prior the intervention and after theinsertion of a guide wire into an arteria as a temporal update. Thereconstruction of the fully sampled (902) and undersampled (904)temporal updates reconstructed using a standard FDK algorithm are shown.In contrast to the undersampled reconstruction 904 that is dominated bystreaking artifacts, the PRIDICT reconstruction 906 that uses a fullysampled prior image and an undersampled temporal update is artifactfree.

It is also possible that the angular positions within each rotation arenot identical to angular positions within other rotations. This can beused to reconstruct more complete data sets (e.g. with a goodsoft-tissue contrast) by combining information from several rotations(FIG. 7).

The imaging system could be able to provide real-time updates of theimaged volume including instrument positions during interventions. Inone embodiment the images are displayed in volume-rendering, surfacerendering or such alike. 3D subtraction angiography can also beperformed by subtracting projections or reconstructed volumes with orwithout contrast media. Furthermore, a 3D road-mapping feature can beintegrated, here the contrasted vessels are superimposed in 3D on timelyupdates to guide the intervention. Furthermore, information of thevessel positions can be integrated into the reconstruction algorithm andcan be used to manipulate the sparsifying function.

The system can be built with several imaging chains, X-ray tubes anddetectors. The angular distance between X-ray tubes can be 90°, forexample. Different X-ray and detector systems can be employed, also inone system. The tube voltage can be varied to provide multi-energyinformation. The multi-energy information can be used to constrain theprior images, to detect changes in the volume (e.g. the differentexpression of guide-wire and tubing in multi-energy systems). The systemcan provide methods to vary the angular distance between projections andthe angular distance over which one projection is acquired. This can bethrough a very fast x-ray on/off method, same kind of shutter or a framework installed within the gantry.

The system can be built so that it can provide all relevant features ofcurrent standard angiography C-arm system, which are X-ray fluoroscopyand digital subtraction angiography. In one embodiment this can be doneby parking the imaging chain at a certain angular position.

In one variation of 4D-CATH the data acquisition is varied according tothe amount of changes or movements in the examined volume. In oneembodiment the differences of projections of several rotations arecompared. In case of little differences the projection acquisition rateor radiation dose is reduced (FIG. 8.). This can also be used tocompensate patient movements that would yield great differences in theacquired projection data and hence require more radiation dose tocompensate for it. This feature could be represented within thereconstruction algorithm as described in FIG. 6/3. In one embodiment theradiologist can influence the effects that the amount of differences hason the reconstruction algorithm or data acquisition (FIG. 9). E.g. byselecting certain programs or modes of the system and by pressing ahandle or a pedal the radiation dose will be much more increased withthe same amount of changes in the examined volume.

In one embodiment the prior scan will be continuously updated by thecontinuous data acquisition. In one current realization the prior willbe generated out of the last 10 rotation consisting of projections thatare acquired at slightly different angular positions.

In one embodiment a spatial-registration algorithm is incorporated thatregisters the prior scan data with the update scan data using standardregistration methods based in the projection or volume domain.

The PRIDICT algorithm can be implemented in various ways using differentsparsifying transforms. One of the simplest implementations is the useof the difference as sparsifying transform. Other possible transformsare e.g. the calculation of the gradient, a wavelet-sparsifying or asparsification using dual energy concepts. As an alternative to onefixed sparsifying function, a tunable function can be used representingdifferent aspects of interventional tools (FIG. 9).

Although embodiments of the present invention have been described withreference to particular examples, it will be clear to a person of skillin the art based on the teachings herein that certain modifications canbe made in the described examples. Throughout the specification, any andall references to a publicly available document, including a U.S.patent, are specifically incorporated by reference.

In one embodiment of the invention the method for performing imagingduring radiological interventions, comprises the steps of measuring morethan one set of projections of an object to be imaged at differentpoints in time, wherein the measurement is performed by an imagingapparatus and dynamic reconstruction of volumetric datasets from themore than one set of projections, wherein the reconstruction isperformed by processing means. In a further embodiment of the invention,the more than one set of projections of the object to be imaged aremeasured in a repetitive or continuous scanning mode of the imagingapparatus. In a further embodiment of the invention, the processingmeans comprise a processor. In yet a further embodiment of theinvention, the more than one set of projections of the object to beimaged comprise undersampled sets of projections. In a furtherembodiment of the invention, the undersampled sets of projections aremeasured at consecutive points in time during the radiologicalintervention. In a further embodiment of the invention, the more thanone set of projections of the object to be imaged comprises at least onefully sampled set of projections, preferably measured before, during orafter the radiological intervention. In a further embodiment of theinvention, the reconstruction is performed by an iterativereconstruction method. In a further embodiment of the invention, theiterative reconstruction method is based on compressed sensing theory.In a further embodiment of the invention, the variations in an algorithmfor the reconstruction are matrix size, interruption criterion,sparsifying functions. In a further embodiment of the invention, theinterruption parameter of the iterative reconstruction method isdepending on the amount of changes in the volumetric datasetsreconstructed from the undersampled sets of projections, the amount ofsignificant pixels and/or the used sparsifying function. In a furtherembodiment of the invention, the more than one set of projections of theobject to be imaged comprise at least one fully sampled set ofprojections and undersampled sets of projections measured at consecutivepoints in time during the radiological intervention, wherein thereconstruction is configured to combine the at least one fully sampledset of projections with undersampled sets of projections. In a furtherembodiment of the invention, imaging parameters of the imaging apparatusdepend on the amount of movement and information changes in anexamination volume. In a further embodiment of the invention, thedependency on the amount of movement and information changes in theexamination volume is influenced by the interventionalist. In a furtherembodiment of the invention, the step of the reconstruction isinfluenced by the amount of changes in the object to be imaged. In afurther embodiment of the invention, a user is provided with some meansto influencing the ratio how changes in the examined volume influencethe data acquisition, reconstruction parameters or data display.

In another aspect of the invention, the method described above is foruse during radiologically guided interventions on the cardiovascularsystem. In a further aspect of the invention, the method described aboveis for use in the implantation of cardiac pacemakers. In a furtheraspect of the invention, the method described above is for use duringradiologically guided interventions on tubular organ structures,preferably lungs or kidneys. In a further embodiment of the invention,for use in positioning of stents in vessels or bronchi. In a furtheraspect of the invention, the method described above is for use duringbronchoscopy interventions. In a further aspect of the invention, themethod described above is for use during catheter interventions. In afurther aspect of the invention, the method described above is for useduring radiologically guided interventions on the brain.

In one embodiment of the invention a system for carrying out the methodas described above, contains an imaging apparatus in communication withprocessing means, wherein the imaging apparatus is a tomographic system,such as a magnetic resonance imaging (MRI) scanner or a computedtomography (CT) scanner. In another embodiment of the invention thecomputed tomography scanner comprises at least one X-ray source and atleast one detector, wherein the X-ray sources differ in terms of theX-ray spectra or the detectors providing means of energydifferentiation.

LIST OF REFERENCE NUMERALS

-   100 Scan system-   101 CT scanner-   102 Array of displays-   103 HPC-   104 Operator control-   105 Operator-   106 Patient-   201 Source-   202 Detector-   203 Object to be imaged-   204 Rotation direction    -   300 Imaging system-   302 Object to imaged-   304 Source-   306 Detector-   308 X-ray-   310 Structure within the object to be imaged-   311, 312 Rotation direction-   400 Flow chart 4D-CATH during the intervention-   402 Catheter placement-   403 Acquisition of update data-   404 Reconstruction of acquired data-   500 Flowchart of 4D-CATH-   501 Acquisition of high resolution CT scan-   502 Performing intervention and acquiring update information-   503 Incorporation of prior image-   504 Reconstruction of image data-   506 Collection of data-   508 Reconstruction of soft tissue image; new prior-   510 Continuous acquisition of low dose tomographic data during    intervention-   514 Situation X-   512 Amount of tomographic data available-   515 Situation X-t-   516 Time axis-   516.1, 516.2 Points in time-   518.1, 518.2 Processing of data available-   520 Reconstruction with further sparsifying function and/or prior    image sparsifying-   521 Reconstruction without further sparsifying function-   524 Display of anatomy-   526 Display of guide wires-   528 Feedback for prior image-   602 Prior image-   604 Update scans-   606 Projection at a solid angle for update scans-   608 Projection at a solid angle for prior image-   610 Incorporation of update scans in prior-   700 PRIDICT reconstruction algorithm-   702 Fully sampled set of projections-   704 FDK-   706 Prior image-   708 Update image-   710 Projector-   712 Set of undersampled projections-   714 Subtraction operation-   716 FDK-   718 Difference reconstruction-   720 Summing operation-   722 Subtraction operation-   724 Image processing, mathematical operations-   800 PRIDICT reconstruction algorithm including minimization-   802 Fully sampled set of projections-   804 FDK-   806 Prior image-   808 Update image-   810 Projector-   812 Set of undersampled projections-   814 Subtraction operation-   816 FDK-   818 Difference reconstruction-   820 Summing operation-   822 Subtraction operation-   824 Image to be minimized-   826 Signum of image to be minimized-   828 Subtraction operation-   830 Comparator CS criterion-   832 Image reconstruction loop-   834 Minimization loop-   900 PRIDICT reconstruction algorithm including sparsifying function-   902 Fully sampled set of projections-   904 FDK-   906 Prior image-   908 Update image-   910 Projector-   912 Set of undersampled projections-   914 Subtraction operation-   916 FDK-   918 Difference reconstruction-   920 Summing operation-   922 Subtraction operation-   924 Image to be minimized-   926 Signum-   928 Subtraction operation-   930 Comparator CS criterion-   932 Image reconstruction loop-   934 Minimization loop-   936 Further sparsifying function-   1000 PRIDICT reconstruction algorithm including influence of    reconstruction parameters-   1002 Fully sampled set of projections-   1004 FDK-   1006 Prior image-   1008 Update image-   1010 Projector-   1012 Set of undersampled projections-   1014 Subtraction operation-   1016 FDK-   1018 Difference reconstruction-   1020 Summing operation-   1022 Subtraction operation-   1024 Image processing, mathematical operation-   1026 Sparsifying function-   1028 Minimization-   1030 Comparison of prior and update scans-   1032, 1034 Influence on reconstruction algorithm-   1040 Flow chart for adapting PRIDICT-   1042 Continuous acquisition of undersampled data-   1044 Calculation of amount of difference/movement-   1048 Variation in reconstruction algorithm-   1050 Providing a marker for the degree of completeness-   1052 Input from radiologist-   1046 Influencing scan parameters reconstruction parameters-   1100 PRIDICT reconstruction algorithm including influence-   1102 Fully sampled set of projections-   1104 FDK-   1106 Prior image-   1108 Update image-   1110 Projector-   1112 Set of undersampled projections-   1114 Subtraction operation-   1116 FDK-   1118 Difference reconstruction-   1120 Calculation of modification-   1122 Adaption of metric size-   1124 Summing operation-   1126 Subtraction operation-   1128 Image to be minimized-   1130 Signum-   1132 Subtraction operation-   1134 Comparator CS criterion-   1136 Image reconstruction loop-   1138 Minimization loop-   1200 Guide wire-   1300 Prior image-   1302 Time frame reconstruction of update-   1304 Time frame reconstruction of prior and update-   1306 Time frame reconstruction of prior-   1308 Heart to be fully imaged-   1310 Cardiac phases-   1312 Projections of prior-   1314 Heart to be imaged through undersampled set-   1316 Projection of undersampled set-   1318 Cardiac phases-   1320 Rotation-   1322 Reconstructed prior-   1324 Reconstructed update in cardiac phase 1-   1326 Reconstructed update in cardiac phase 2-   1328 Reconstructed update in cardiac phase 3-   1330 Instrument-   1332 Prior incorporated into each reconstruction of cardiac phases-   1334 Reconstructed prior in cardiac phase 1-   1336 Reconstructed prior in cardiac phase 2-   1338 Reconstructed prior in cardiac phase 3-   1340 Reconstructed update-   1341 Instrument-   1342 Prior for cardiac phases incorporated into reconstruction-   1344 Reconstructed prior in cardiac phase 1-   1346 Reconstructed prior in cardiac phase 2-   1348 Reconstructed prior in cardiac phase 3-   1350 Reconstructed update in cardiac phase 1-   1352 Reconstructed update in cardiac phase 1-   1354 Reconstructed update in cardiac phase 1-   1356 Instrument-   1358 Prior for cardiac phases incorporated into reconstruction of    update for cardiac phases-   1410 Full dose prior-   1412 Low dose update-   1414 4D data set of moving instruments-   1416 4D intervention guidance in 3D road map-   1418 Collected Low dose updates-   1420 3D data set of vascular

1. An imaging method for radiologically guiding an instrument duringmedical interventions on an object, the method comprising: a) providinga first image of said object followed by b) providing updated imageson-the-fly during the intervention to an operator by measuring anundersampled set of projections of said object and reconstructing saidupdated image based on changes between said first image or an update ofsaid first image and said undersampled set of projections.
 2. Methodaccording to claim 1, wherein said imaging method is based on ionizingradiation and the undersampled set of projections is measured with aminimized radiation dose.
 3. Method according to claim 1, wherein saidfirst image, said update of said first image and said updated imagescomprise volumetric images of the object.
 4. Method according to claim1, wherein said first image comprises a high-resolution volumetric imageof the object, which is acquired prior to the intervention, optionallybased on a fully sampled set of projections, or which is provided from adatabase.
 5. Method according to claim 1, wherein said update of saidfirst image encompasses said first image and at least parts of one ormore updated images reconstructed in previous runs during theintervention.
 6. Method according to claim 1, wherein said update ofsaid first image is a sliding prior defragmenting updated image ofprevious runs.
 7. Method according to claim 1, wherein said undersampledsets of projections measured on-the-fly are stored for a delayedreconstruction of soft tissue contrast.
 8. Method according to claim 1,wherein the amount of said changes is determined by forward projectingsaid first image or said update of the first image according to theprojections comprised in the undersampled set of projections giving aset of equivalent projections based on the said first image or saidupdate of the first image and subtracting equivalent projections. 9.Method according to claim 1, wherein the reconstruction includes aniterative minimization of a number of significant pixels optionallyincluding at least one further sparsifying function.
 10. Methodaccording to claim 9, wherein the at least one further sparsifyingfunction included in the reconstruction depends on the amount of datathat has been acquired before the actual present.
 11. Method accordingto claim 10, wherein updated images are reconstructed with no furthersparsifying function and wherein the reconstruction of data stored fromprevious runs includes at least one sparsifying function.
 12. Methodaccording to claim 1, wherein the reconstruction is influenced by theamount of said changes between measurements.
 13. Method according toclaim 1, wherein the number of projections in the undersampled set ofprojections and the dose rate per projection are adapted according tothe amount of changes between measurements and/or according to an inputprovided by the operator.
 14. Method according to claim 1, wherein thereconstruction of said updated includes motion compensation.
 15. Methodaccording to claim 1, wherein periodic and/or non-periodic motion of theobject is compensated for in the reconstruction of said updated images.16. Method according to claim 15, wherein periodic motion compensationis performed through gating images into different phases of saidperiodic motion.
 17. Method according to claim 15, wherein periodicmotion compensation is performed through a transformation mapping saidupdate images and/or said first image into one phase of said periodicmotion.
 18. Method according to claim 1, wherein said updated images aredisplayed on a screen allowing for different representation modes, whichare chosen automatically or by said operator.
 19. Method according toclaim 1 for use in 4D radiological guidance.
 20. Method according toclaim 1 for use during interventions on the cardiovascular system, foruse during catheter interventions and/or for use in the implantation ofcardiac pacemakers.
 21. Method according to claim 1 for use duringinterventions on tubular organ structures, preferably lungs or kidneys,for use in positioning of stents in vessels or bronchi or for use duringbronchoscopy interventions.
 22. Method according to claim 1 for useduring interventions on the brain.
 23. A system for radiologicallyguiding medical interventions on an object according to the method ofclaim 1, the system comprising: means to provide a first image of theobject; an imaging apparatus for measuring undersampled sets ofprojections; processing means in communication with the imagingapparatus for providing updated images on-the-fly during theintervention by reconstructing said updated image based on changesbetween said first image or an update of said first image and saidundersampled set of projections.
 24. The system according to claim 23,wherein the imaging apparatus is a tomographic system, such as amagnetic resonance imaging (MRI) scanner or a computed tomography (CT)scanner.
 25. The system according to claim 24, wherein the computedtomography scanner comprises at least one X-ray source and at least onedetector.
 26. The system according to claim 25, wherein the X-raysources differ in terms of the X-ray spectra or the detectors providingmeans of energy differentiation.
 27. The system according to claim 23,wherein imaging parameters of the imaging apparatus depend on thechanges between said first image or an update of said first image andsaid undersampled set of projections.
 28. The system according to claim23 wherein the system includes means to provide an output to anoperator, the output comprises at least one updated image, at least oneimage produced by delayed reconstruction of soft tissue contrast and/oran indicator for an accuracy of displayed images.
 29. (canceled) 30.(canceled)